As a cutting tool with detachable insert used for grooving or cutting-off of a work material, there is a tool having the following structure. In this tool, a pressing surface is provided in one jaw portion of a pair of jaw portions formed at the tip of a holder. And a pedestal surface is provided in the other jaw portion of the pair of jaw portions. The pressing surfaces and a pedestal surface are arranged so as to face each other. An insert attachment seat is formed in this way. Additionally, from a rear end of the insert attachment seat, a slit is provided so as to further extend towards the rear end of the tool. A cutting insert which has cutting blades is clamped and attached between the pressing surface and the pedestal surface by screwing in the clamp screw engaged with one jaw portion, and elastically deforming one jaw portion. In Patent Document 1, a cutting tool with detachable insert in which this insert attachment seat is formed in a head member detachably mounted on the tip portion of the holder is suggested.    [Patent Document 1] Japanese Patent Unexamined Publication No. 05-192802